linux-kernel - Re: [PATCH v5 03/23] PM: EM: Find first CPU active while updating OPP efficiency

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Message-ID: <20231217175829.a6hqz7mqlvrujsvs@airbuntu>
Date: Sun, 17 Dec 2023 17:58:29 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Lukasz Luba <lukasz.luba@....com>
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	rafael@...nel.org, dietmar.eggemann@....com, rui.zhang@...el.com,
	amit.kucheria@...durent.com, amit.kachhap@...il.com,
	daniel.lezcano@...aro.org, viresh.kumar@...aro.org,
	len.brown@...el.com, pavel@....cz, mhiramat@...nel.org,
	wvw@...gle.com
Subject: Re: [PATCH v5 03/23] PM: EM: Find first CPU active while updating
 OPP efficiency

On 11/29/23 11:08, Lukasz Luba wrote:
> The Energy Model might be updated at runtime and the energy efficiency
> for each OPP may change. Thus, there is a need to update also the
> cpufreq framework and make it aligned to the new values. In order to
> do that, use a first active CPU from the Performance Domain. This is
> needed since the first CPU in the cpumask might be offline when we
> run this code path.

I didn't understand the problem here. It seems you're fixing a race, but the
description is not clear to me what the race is.

> 
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  kernel/power/energy_model.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index 42486674b834..aa7c89f9e115 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -243,12 +243,19 @@ em_cpufreq_update_efficiencies(struct device *dev, struct em_perf_state *table)
>  	struct em_perf_domain *pd = dev->em_pd;
>  	struct cpufreq_policy *policy;
>  	int found = 0;
> -	int i;
> +	int i, cpu;
>  
>  	if (!_is_cpu_device(dev) || !pd)
>  		return;
>  
> -	policy = cpufreq_cpu_get(cpumask_first(em_span_cpus(pd)));
> +	/* Try to get a CPU which is active and in this PD */
> +	cpu = cpumask_first_and(em_span_cpus(pd), cpu_active_mask);
> +	if (cpu >= nr_cpu_ids) {
> +		dev_warn(dev, "EM: No online CPU for CPUFreq policy\n");
> +		return;
> +	}
> +
> +	policy = cpufreq_cpu_get(cpu);

Shouldn't policy be NULL here if all policy->realted_cpus were offlined?


Cheers

--
Qais Yousef

>  	if (!policy) {
>  		dev_warn(dev, "EM: Access to CPUFreq policy failed\n");
>  		return;
> -- 
> 2.25.1
>